1. Field of the Invention
The present invention relates to a method for determining the rotational position of the drive shaft of a direct current (DC) motor by evaluating the current ripple contained in the armature current signal.
2. Background Art
The armature current signal of a direct current (DC) motor includes a so-called direct component and a ripple component superimposed on the direct component. The ripple component arises when the DC motor is operated as a consequence of the interaction of the magnetic field, the armature winding, and the commutator of the DC motor. This expresses itself in a short-term change in the induced voltage, which produces the ripple content of the armature current signal. The current peaks contained in the armature current signal (referred to below as “current ripple”) occur when the armature rotates, and have a frequency corresponding to the number of collector bars.
For example, if the armature has ten collector bars, the armature current signal accordingly has ten current ripples that can be identified in it. Thus, counting the current ripples can give information about the current rotational position of the armature of the DC motor and thus also regarding an element driven by it within a predetermined segment of travel. To accomplish this the analog armature current signal is digitized to be able to perform a corresponding counting.
Such processes are used, for example in the area of motor vehicles to control adjustment drives, such as are provided for power windows and/or sunroofs, for example. An essential element in capturing the position of the window glass, for example, is the position at which the pinching protection can be turned off when the window is closed. The pinching protection has to be turned off so that the window glass can go completely in its top block and into the seals provided there, without the motor being turned off as a result of increased load. If the counting of the current ripples to determine the position of the window glass is incorrect, it can happen that the pinching protection is turned off too early or too late.
However, such a process is subject to inaccuracies in determining the exact position due to the DC motor starting when power is supplied to it and slowing down when it is turned off. These inaccuracies result from the fact that when a DC motor starts, its armature has to rotate a certain amount before the first current ripple is detectable. The same goes for when the motor slows down, with the conventional process not being able to detect current ripples after the DC motor is turned off, so that the movement of the armature in the motor slowdown phase cannot enter into the position determination.